Today, information recording technology, particularly, magnetic recording technology is needing a drastic technical innovation, attendant on the progress of the so-called IT industry. In magnetic disks loaded in hard disk drives (HDD) functioning as magnetic disk apparatuses, rapid increases in information recording density are being made, unlike the cases of other magnetic recording media such as magnetic tapes and flexible disks.
Such a magnetic disk has a magnetic layer or the like formed on a substrate such as an aluminum alloy substrate and a glass substrate. In a hard disk drive, while a magnetic head is kept flying over a magnetic disk being rotated at a high speed, an information signal is recorded into the recording layer as a magnetization pattern or is reproduced from the recording layer by this magnetic head.
In recent years, in such magnetic disks, the information recording density has come to exceed 40 Gbits per square inch, and, further, a superhigh recording density of more than 100 Gbits per square inch is about to be realized. The recent magnetic disks with which a high information recording density can be realized as above have a feature such that a practically sufficient amount of information can be stored therein notwithstanding the much smaller disk areas, as compared with the flexible disks and the like magnetic disks according to the related art.
In addition, the magnetic disks under consideration have also a feature such that the information recording speed and reproduction speed (response speed) are extremely high, as compared with those of other information recording media, and that information can be written and read at any time desired.
As a result of the attention paid to the various features of the magnetic disks, in recent years, there has been a demand for a small-size hard disk drive capable of being mounted in portable apparatuses which are much smaller in housing size than personal computers and in which a high response speed is demanded, such as cellular phones, digital cameras, portable information equipment (e.g., PDA (personal digital assistant)) and car navigation systems.
Along with the increased demand for mounting of a hard disk drive into a portable apparatus (i.e., for the so-called “mobile use”), a glass substrate composed of a glass, which is a hard material, has been adopted as a substrate for the magnetic disk. This is because glass substrates are higher in strength and rigidity than substrates composed of a metal, which is a soft material.
Besides, in the glass substrates, a smooth surface can be obtained. By use of a glass substrate, therefore, it is possible to obtain a magnetic disk with which a decrease in the flying height of the magnetic disk operative to perform recording and reproduction while flying over the magnetic disk (a lowered flying height) can be realized together with a high information recording density, while preventing such troubles as head crash and thermal asperity.
The glass substrates, however, also have an aspect that the glass is a brittle material. In view of this, there have been proposed various methods for strengthening glass substrates. For example, Patent Document 1 describes a chemical strengthening treatment in which a glass substrate is immersed in a melted liquid of a nitrate, such as sodium nitrate (NaNO3) and potassium nitrate (KNO3), heated at about 300° C. in a chemical strengthening tank for a predetermined time, whereby lithium ions (Li+) in surface layer parts of the glass substrate are replaced by sodium ions (Na+) or potassium ions (K+), or sodium ions (Na+) in surface layer parts of the glass substrate are replaced by potassium ions (K+), and compressive stress layers are formed at the surface layer parts on both sides, with a tensile stress layer being formed between the compressive stress layers.
In addition, Patent Document 2 describes a method for performing a chemical strengthening treatment while suppressing warping of a glass substrate, in which a tiny amount of a cationic impurity is mixed into a nitrate melted liquid used in the chemical strengthening treatment so as to regulate the complete solidification point of the nitrate to 130° C. or below, thereby ensuring that the nitrate shows fluidity when cooled after the chemical strengthening treatment.    Patent Document 1: JP-A-2002-121051    Patent Document 2: JP-A-2001-192239